It's not that they made more money from merchandise, it's that they sold more t-shirts than albums. Implying that more people were interested in the "image" of punk rock than the music.
I can wear out a t-shirt much faster than an album, tape or CD, and I am not very caring of the conditions of albums.
I've also never seen anyone slam dance carrying a Ramones album, but I have seen them slam dance wearing a Ramones t-shirt that got tore up.
The last time I went to an Exodus show, there were more people in attendance than sales of their most recent album.
Although the article is unsure whether they sold more t-shirts than tickets, implying that people were interested in the music in a live capacity.
Which is a reasonable implication given that punk grew up around the DIY culture. A commercially produced recording doesn't exactly align with the interests of that type of community, even where that community enjoys the music itself.
I guess that's the definition of 'iconic' - many a time I have approached someone wearing a Ramones or Motörhead T-shirt trying to chat a bit, only to be told 'Sorry, don't know the music at all, but the shirt is cool...'
Gabba gabba hey!